Fuel for the propulsion and position control thrusters of a spacecraft or missile is pressurized in at least one fuel tank which supplies the fuel under pressure into a reaction chamber. The fuel is either liquid or gaseous. Propulsion gases are produced in the reaction chamber, either by a chemical reaction of the fuel and/or by a decomposition. Due to the continuous removal of fuel from the fuel tank, the pressure in the fuel tank also continuously decreases.
The operation of spacecraft thrusters with a continuously decreasing pressure in the fuel tank is known as a blow-down operation. Such operations take place primarily as continuous operations. As a result, the thrust produced by the thruster becomes smaller and smaller as the operation continues and the fuel supply pressure decreases. The reason for this decrease is due to the fact that the fuel tank or in the case of a two component fuel, the fuel tanks, which are filled with the fuel or the fuel components, are charged with a compressed gas only once for pressurizing of the fuel or fuel components. As a result of the removal of the fuel from the fuel tank, the pressure sinks in the fuel tank and never returns to its initial value. As a result, the fuel mass flow per second or throughput through the thruster also decreases which in turn results in a reduction of the efficiency of the thruster. Incidentally, the term "throughput" as used herein means mass flow per second. Due to this pressure reduction, the in fact achievable optimal specific impulse value for such thrusters is achieved only for a relatively short time compared to the entire combustion or operational time of such thrusters.
The above described effect becomes especially disadvantageous in those cases in which a number of nominally identical thrusters are to be used, for example, for the position control of the missile or spacecraft.
German Patent Publication (DE-OS) 3,128,735 (Steenborg), published on Feb. 10, 1983 discloses a thruster which operates at increased temperatures due to the catalytical decomposition of a liquid energy carrier, whereby a closed loop control of the produced thrust is achieved in such a manner that the decomposition reaction and thus the quantity of fuel gas produced per unit of time, is controlled by a change of the temperature in the decomposition or reaction chamber. The temperature control is achieved by an electronic control circuit which controls the electrical current flowing through respective heating elements.